Primary Treatment of Wastewater Definition
Primary Treatment Wastewater is a plain sedimentation process to remove suspended organic solids from the sewage. Chemical are sometimes used to remove finely divided and colloidal solids.
Objectives of Primary Treatment
The main objectives of primary treatment of wastewater are:
- To reduce the strength of sewage to the extent 30% to 50%.
- To remove settleable solids by 80% to 90%.
- To reduce BOD by 30% to 35%.
- To make the sewage fit for further treatment process.
Primary Sedimentation Tank
Primary sedimentation tank is also known as primary clarifier and is located just after grit chamber. It may be rectangular, circular or square shape. The principle and construction details are same as that of plain sedimentation tank W.T.P.
Design Specifications of Primary Sedimentation Tank
Following steps are involved in pimary treatment wastewater design
- Hydraulic loading rate (surface overflow rate)/settling velocity Vs = (0.3 – 0.7) mm/sec (1 – 2.5 m/hr)
- Detention time / retention time Td = 1 – 2 hrs
- Tank Depth = (1 – 5) m
- BOD removal (20 – 40) %
- Suspended solids removal (30 – 60) %
- Minimum number of tanks = 2
- Sludge accumulated = 2.5 Kg of wet solids / m3 of flow.
Types of Primary Sedimentation Tanks
- Typical primary sedimentation tank
- Circular Radial Flow Tank
- Up Flow Tanks